1. Field of the Invention
This invention relates to a four-stroke engine for a motorcycle.
2. Description of Related Art
A conventional type of water-cooled engine uses a structure having a cooling jacket for surrounding peripheries around a combustion chamber of a cylinder head and a cylinder in a cylinder block in order to ensure even temperature distribution. However, the heat load is not distributed evenly over the peripheries around the combustion chamber and the cylinder. Therefore, some areas are overcooled.
A water-cooled engine has a cooling system with a maximum heat radiation performance, that is, sizes of a radiator and a water pump, designed such that a long life coolant (LCC) as a refrigerant exceeds not more than a preset boiling point. Thus, the sizes of the radiator and the water pump tend to be designed large enough, or even overlarge, relative to a desired temperature in areas to be cooled.
A type of water-cooled engine, which has a cooling jacket formed on the cylinder head and the cylinder block, and cooling fins, as well as another type, which supplies cooling wind by a cooling fan to areas around the cylinder head and the cylinder block, are known.
In contrast to that, an air-cooled engine cannot be needed to have the radiator, water pump and cooling water line. This allows the engine size to be reduced while improving design flexibility in the engine and the body.
The air-cooled engine functions well except in the case of an extremely high heat load. The engine has a problem with an abnormal rise in temperature especially around an exhaust port, as the engine displacement increases, affecting the engine's output to some extent.
The air-cooled engine may additionally have a partial water cooling system for water cooling an area subject to an extremely high heat load. This type of air-cooled engine requires auxiliary devices such as the radiator and water pump to be disposed. This may cause an increase in the size of the engine, and limit the design flexibility specific to the air-cooled engine, depending on a predetermined area to be water-cooled, a size of each auxiliary device, and their arrangement.
In view of the foregoing, an advantage of this invention is to provide a four-stroke engine which can prevent an increase in size of the engine with a water cooling system, and ensure design flexibility in the engine and body.